Microwave based RF communication devices are known. Typically, in communication devices that operate above 2 GHz, transmitter and receiver oscillators are comprised of metal encapsulated cavities that serve to stabilize a free running, active oscillating device such as a Gunn diode. In general, such cavities will support oscillation of the active device at a single frequency, which frequency will be determined by the inside dimensions of the cavity, the dimensions and material of purposely inserted tuning element, and to some extent, the characteristics of the load.
Through the use of tuning elements that can be inserted into the cavity, the frequency can be preselected very accurately as appropriate to accomodate a given system's requirements. Unfortunately, temperature variations will cause this frequency to drift. In particular, the material that defines the cavity will expand or contract slightly with temperature changes. Also, the active device itself will vary its activity somewhat with temperature variations, as will the load characteristics. All of these changes cause a resultant change in the oscillation frequency, and thereby degrade the performance of the device.
To deal with this problem in microwave cavity oscillators, the prior art discloses two alternative solutions. The first, and more popular approach, is to specifically design the oscillator for a given tightly limited frequency range, through choice of materials and mechanical connections. In this way, an oscillator can be designed to remain acceptably stable over a workable temperature range. The great drawback to this approach is the limited tuning range of the resultant oscillator. In addition to increased costs of design and construction to match each desired tuning range to acceptable temperature performance, the manufacturer, distributors, and users of such oscillators must maintain a large inventory of oscillators, since they must be able to accommodate needs over a wide frequency range.
The second prior art approach requires external frequency detection circuity to monitor for drift and to use this information in a feedback loop to control the frequency of oscillation accordingly. This approach represents an expensive alternative, and has found little use to date in many frequency bands above 20 GHz.
A need therefore exists for a broadband temperature compensated microwave cavity oscillator that can be tuned over a wide tuning range, and that can be easily temperature compensated to remain accurate within that tuning range. As an example of the accuracy required, devices intended for operation in the 21.2 GHz to 23.6 GHz radio band can drift no more than plus or minus 0.03% over an ambient operating temperature range of minus 30 degrees C. to positive 50 degrees C.